1. Field of the Invention
The invention relates to an audio signal noise reduction device that is equipped in a digital private electrical appliance, for example, and reduces the noise level of an audio signal captured by a small-sized microphone, and an audio signal noise reduction method.
2. Description of the Related Art
Digital private electrical appliances each including therein a small-sized microphone, e.g., video cameras, digital cameras, and IC recorders, are becoming increasingly smaller in size. Such a smaller-sized appliance often causes problems of arising unpleasant shock noise, touch noise, or click noise at the time of audio reproduction. Such noise is caused by frequent accidental touch on a microphone and therearound at the time of capturing audio, or click operations of various function switches SW, resulting in the noise entering the microphone after being transmitted through a cabinet. There is another problem caused by the proximity between the internal microphone and a recording device, e.g., tape unit and disk unit, equipped in the private electrical appliance. Such proximity possibly causes noise of the recording device, e.g., vibration noise and acoustic noise, to enter the microphone.
For the aim of reducing such noise, in the internal microphone, a microphone unit is made to be suspended using an insulator such as rubber damper, or the microphone unit is made to be floated using a rubber wire or others. With such a configuration, any vibration coming from the cabinet is absorbed so that no noise is transmitted to the microphone unit. However, this configuration is not enough to achieve the vibration-free environment, i.e., the insulator does not work right if with strong vibration or some vibration frequency, or the resonant vibration occurs with some unique frequency. As such, designing such a configuration is difficult, being the factors that have been hindering the cost saving or size reduction.
For betterment, various other noise reduction methods have been proposed, however, none is satisfactory in terms of meeting users' asking level. This is because the above-described noise includes not only the vibration noise coming from the cabinet but also the acoustic noise that is transmitted as sound in air together with the vibration. Such a noise complicates the transmission path of the noise toward the microphone unit, thereby resulting in a limitation of noise reduction with the previous passive methods.
For the same purpose, the applicant of the invention has proposed the noise reduction technology in Patent Document 1 (JP-A-2005-57437; Microphone Unit, Noise Reduction Method, and Recording Device). In Patent Document 1, the noise reduction is implemented by generating a pseudo noise signal using an adaptive filter, and subtracting the pseudo noise signal from a noise-included audio signal.